Parallel-connected switching devices can be used in applications where high current throughput is specified. The parallel connection is used in inverter bridges feeding high current AC- or DC-loads.
A known problem associated with the parallel-connected switching devices is that currents through the switching devices are seldom evenly balanced. FIG. 1 illustrates examples of uncompensated schematic current waveforms for three parallel-connected IGBTs in accordance with known systems. The waveforms show unbalanced current distribution due to differentiating turn-on and turn-off times. Unbalanced currents cause uneven wear and tear in the switching devices. The uneven wear and tear in turn reduces the lifetime of the switching devices. Further, unbalanced currents may cause a switching device to reach its maximum temperature rating or maximum current rating faster than when the currents are in balance.
In known implementations, parallel-connected power electronic switching devices can be derated or additional passive components can be added in the current path to reduce the current imbalance, as disclosed in publication B. Abdi, A. H. Ranjbar, K. Malekian, J. Milimonfared, and G. B. Gharehpetian, “Problems associated with parallel performance of high current semiconductor switches and their remedy,” in Proc. Int. Symp. Power Electronics, Electrical Drives, Automation and Motion SPEEDAM 2008, 2008, pp. 1379-1383. However, this approach can be inefficient and costly.
Selection of a component with similar characteristics can also be used as a tool to solve the problem, as disclosed in publication Fuji IGBT module application manual, www.fujisemiconductor.com, 2004. However, handpicking the components adds production costs.
In a publication by Dominik Bortis, Juergen Biela, Johann W. Kolar, “Active gate control for current balancing of parallel-connected IGBT modules in solid-state modulators”, IEEE Transactions on Plasma Science, vol. 36, no. 5, pp. 1379-1383, an active current balancing by gate controlling circuits is proposed. This publication discloses utilization of centralized intelligence only. The disclosed method requires a relatively complex measurement and communication system to operate.